A large meteorite smashed into what is now Santa Fe, N.M., some million or billion years ago. Now scientists are trying to determine exactly when this life-killer hit. Space.com reports that geologists are studying the impact crater in detail to determine its size and moment of earth-shattering impact.

The Southwest is home to other meteor impact craters. Just across state lines is the Meteor Crater of Arizona, which is known as one of the best-preserved meteor sites on Earth. The Meteor Crater, located about 43 miles east of Flagstaff, is about 4,000 feet (1,200 meters) wide and 570 feet (170 meters) deep. It hit some 50,000 years ago during thePleistocene epoch when mammoths and giant ground sloths roamed the open plains. People were thought to appear much later in the area.

But the meteor that wiped out what is now Santa Fe is thought to have been much bigger than the one that struck Arizona. And much of it remains a mystery. As Space.com reports, geologists have only been able to determine that the meteorite struck sometime between 1.2 billion and 330 million years ago. Figuring out the exact age of the impact has remained a mystery to experts.

Age and other important facts about the strike have remained elusive largely because of “The Great Unconformity.” An unconformity in geological terms represents a time when no rocks were preserved. And in the Southwest, a billion years of geology was destroyed by rising and falling area sea levels. Rocks were exposed and then hidden, creating a mishmash of geology under the Earth’s surface.

As a result, a 330-million-year-old rock layer now lies on top of rocks over 1 billion years old. This makes study of the Santa Fe crater difficult. A history of excessive volcanic activity in the Southwest has also complicated research of this area. Volcanoes and meteors are generally considered the most cataclysmic ways the Earth can be reshaped.

Nonetheless, experts think the Santa Fe meteor created an impact of 6 to 13 kilometers in diameter. Further, a core of rock dug up in this area is proving exciting to geologists. (A core is cylinder of rock that informs scientists as to what the Earth looks like below.) Horton Newsom is a geologist at the University of New Mexico's Institute of Meteoritic. As he told Space.com of the sample, "Such impact crater cross-sections are extremely rare in the world.”

Newsom and his team of experts will continue to study the Santa Fe crater, though Newsom notes, "It could take several lifetimes to do all the necessary work."